Japanese Prime Minister Shinzo Abe's ruling party suffered an embarrassing potential setback this week when an expert it called to comment in parliament said legal changes to let troops defend an ally under attack would violate the constitution.

Japanese Prime Minister Shinzo Abe's ruling party suffered an embarrassing potential setback this week when an expert it called to comment in parliament said legal changes to let troops defend an ally under attack would violate the constitution.

Abe's cabinet last year adopted a resolution reinterpreting the pacifist constitution to let Japan exercise its right of "collective self-defence", or militarily aiding an ally under attack. The change is part of Abe's more muscular defence policy that would give Japan a bigger role in the US-Japan alliance.

The ruling bloc aims to pass bills to implement that change - which would drop a long-standing ban on troops fighting overseas - in the current session of the parliament and has been expected to do so, given its majority in both chambers.

But Waseda University's constitutional expert Yasuo Hasebe, called by Abe's Liberal Democratic Party (LDP) to comment in a parliamentary panel on Thursday, shocked his hosts by saying the change would violate the constitution. The view was echoed by two other experts invited by opposition parties.

"I think allowing the use of the right of collective self-defence violates the constitution," Hasebe said in response to a question from an opposition lawmaker."It cannot be explained within the framework of the basic logic of the past government views ... and considerably damages legal stability."

Government officials quickly rejected the view but opposition lawmakers seized on the comments to criticise the government during a debate on the planned legislation on Friday.

"It looks like what (defence) minister (general) Nakatani is saying is deceiving the people," said Kiyomi Tsujimoto of the opposition Democratic Party, after Nakatani defended the bills.

The legislation has been expected to pass in a session of parliament that could be extended through August. But some analysts said enactment could be delayed, if not derailed.

"The LDP scored an 'own goal' . The 'incident' has not only flared up in cyberspace, but is also reported widely by mainstream media," said Sophia University professor Koichi Nakano. "It is now conceivable that the legislative agenda will seriously be delayed."

Voters are confused and divided over the proposed changes.

Some 80% of respondents to a Kyodo news agency poll last month said the government's explanation of legislation to implement the security policy shift was insufficient. Nearly 48% opposed the bills while 35.4% were in favour.